1. Field of the Invention
This invention relates to ultrasonic receiver systems and more specifically to a laser-based, self-referencing, non-contacting ultrasonic receiver system.
2. Description of the Related Art
Ultrasonic waves are commonly used to probe a variety of materials, particularly for flaw detection. In typical flaw detection systems, the ultrasonic waves are used to probe the workpiece. The waves reflect or scatter from an internal feature or flaw and propagate to a surface of the workpiece, causing the surface to vibrate.
Optical ultrasonic detection techniques, such as those described in C. B. Scruby and L. E. Drain, Laser Ultrasonics, Techniques and Applications, Adam Hilger, New York (1990), pages 325-350, are used to remotely detect the ultrasonic waves at the surface of the workpiece. Generally, a laser beam is reflected or scattered from the vibrating surface of the workpiece. The vibrating surface imparts a phase shift to the reflected beam, which is then optically interfered with a reference beam that originates from the same laser source as the reflected beam. The beat frequency between the reflected and reference beams corresponds to the surface motion.
One problem associated with laser detection systems is the presence of extraneous acoustic noise sources which cause additional vibrations at the surface. These additional vibrations are picked up by the reflected laser beam and reduce the signal-to-noise ratio of the system.
Another problem associated with optical systems is low sensitivity. Typically, the surface of the workpiece that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. Since the optical interference efficiency between the reflected beam and the reference beam depends, in part, upon how well their wavefronts match, the sensitivity of the system is low for diffusely scattering surfaces.
One prior laser based ultrasonic detection system, described in U.S. Pat. No. 5,131,748, entitled "BROADBAND OPTICAL DETECTION OF TRANSIENT MOTION FROM A SCATTERING SURFACE BY TWO-WAVE MIXING IN A PHOTOREFRACTIVE CRYSTAL", issued Jul. 21, 1992 to Jean-Pierre Monchalin, et. al., addresses the wavefront matching problem. In this system, a laser beam is reflected from a vibrating surface of a workpiece, which imparts a phase shift onto the reflected beam that corresponds to the amplitude and frequency of the surface vibration. The reflected beam is caused to optically interfere inside a photorefractive crystal with a "pump" beam that is derived from the same laser as the reflected beam. The two beams write an index of refraction grating inside the crystal that diffracts the pump beam in the propagation direction of the reflected beam. When the diffracted pump beam and the reflected beam exit the crystal, they are overlapping and have substantially matching wavefronts. However, the index grating matches the phases of the diffracted pump beam and the reflected beam.
In interferometry, the sensitivity of the system is maximized-by-biasing the two interfering beams so that they have a .pi./2 phase shift between them. Since the phases of the two interfering beams in the Monchalin system are matched, its sensitivity to the small phase perturbations imparted to the reflected beam by the surface vibrations is very small. To overcome this problem, a second frequency shifted pump beam is superimposed onto the first pump beam. The second pump beam is close enough in frequency to the first pump beam to be Bragg matched to the index grating and, therefore, diffracts off this grating. A second index grating is not written by the second pump beam and the reflected beam because the crystal cannot respond fast enough to the moving fringe grating produced between the beams (the frequency shift between the beams results in non-stationary fringes). As a result, the second pump beam only diffracts off the first stationary grating (written by the first pump beam and the reflected beam) and the relative phase between it and the reflected beam is preserved.
Although this technique improves the system's sensitivity, it suffers from many limitations. First, the second pump beam has to be Bragg matched to the stationary grating written by the first pump beam and the reflected beam. As a result one cannot impart the frequency shifts needed to operate the system in a heterodyne mode. Second, although the wavefronts of the reflected beam and the diffracted pump beam are matched, they are matched to the aberrated wavefront of the reflected beam rather than to the clean wavefront of the pump beam. For example, if the reflected beam is highly diverging, the diffracted pump will likewise be highly diverging. This could lower the amount of light available to the optical detectors in the system. Third, if the surface of the workpiece is de-polarizing (either locally or globally), the sensitivity of the detector goes down. In addition, if the workpiece surface contains highly contrasting features (for example, pits, rust, spots, etc.), the two-wave mixing amplification may result in non-uniform "print-through" (due to pump depletion) which will degrade the system performance. Finally, the Monchalin system does not compensate for extraneous acoustic noise sources which could cause additional vibrations at the surface. These additional vibrations would be detected by the Monchalin system and would lower the signal-to-noise ratio.
A profilometer, described in M. J. Offside, M. G. Somekh and C. W. See, "Common Path Scanning Heterodyne Optical Profilometer for Absolute Phase Measurement", Applied Physics Letters, Vol. 55, No. 20 (1989), pp. 2,051-2,053, utilizes an interferometric detection system to measure the surface profile of a workpiece. A probe beam is reflected off a small region of a surface of a workpiece and optically combined and interfered with a global reference beam, which is reflected from a different region of the workpiece surface, at a detector. The surface profile is determined by measuring the phase difference between the beams. Phase errors imparted onto the probe beam due to microphonics in the workpiece are compensated by reflecting a compensation beam off a much larger area of the workpiece surface than the probe beam and interfering it with the global reference beam.
Although the Offside system is suitable for surface profiling, it suffers from limitations that make it undesirable as an ultrasonic receiver. First, the system does not compensate for wavefront or polarization distortions imparted onto the reflected beams (from rust spots, absorbing patterns on the surface, cracks, pits, etc.), resulting in reduced sensitivity as explained above. In addition, the system is set up in the form of two Michelson interferometers which require precise alignment of the beams. Any slight misalignment of the beams will lower the system's efficiency.
A dual-probe interferometer for detection of acoustic surface waves is described in Jin Huang and J. D. Achenbach, "Dual-Probe Laser Interferometer", J. Acoust. Soc. Am., Vol. 90, No. 3 (September 1991), pp. 1269-1274. However, this system does not function well with rough surfaces that widely scatter the incident laser beams. In addition, although it compensates for piston noise vibrations, it does not compensate for tilt noise vibrations, dynamic depolarization and dynamic variations in the collected light levels (due to changes in surface absorptions, scattering, etc.).